This invention relates to a photodiode, and more particularly to a method of manufacturing a photodiode having a light-receiving shallow p-n junction.
The term "photodiode" is used in this specification to mean a light-receiving diode and includes a solar cell. A semiconductor photodiode includes a p-n junction in the neighborhood of its light receiving surface. Light incident on the light receiving surface of the semiconductor photodiode is partly reflected by the light receiving surface, but the remainder penetrates into the semiconductor substrate. The semiconductor substrate has an absorption coefficient which is a function of the wavelength of the light, and the incident light is absorbed according to the absorption coefficient. Therefore, the intensity of the light incident on the surface of the semiconductor substrate is exponentially attenuated toward the interior of the semiconductor substrate. Further, the absorption coefficient is larger for light of short wavelengths, i.e. high energy, than for light of long wavelengths. Therefore, the light of short wavelengths is attenuated faster than the light of other wavelengths. Absorption of light having energy higher than the band width causes generation of electron-hole pairs. Besides diffusion, photoexcited carriers migrate due to, for example, drift attributable to the presence of a built-in electric field. When the holes (electrons) reach the anode (cathode) as a result of migration, photoexcited current can be obtained. However, no electric power can be derived from the semiconductor photodiode to the exterior when recombination of the holes and electrons occurs in the midway of their migration.
Considering the exponential attenuation of the intensity of incident light from the surface of the semiconductor substrate toward the interior of the semiconductor substrate, the p-n junction is preferably formed in the neighborhood of the semiconductor substrate surface (that is, as shallow as possible).
The mechanism of recombination determining the life time of the carriers is roughly classified into surface recombination and bulk recombination. Both the former and the latter contribute to the recombination of the holes and electrons in the neighborhood of the light receiving surface of the semiconductor substrate, while the latter contributes mainly to the hole-electron recombination inside the semiconductor substrate.
The p-n junction is preferably as shallow as possible in order that light (especially, light of short wavelengths) can be efficiently converted into a photoexcited current. Generally, in the case of a solar cell of silicon, the depth of the p-n junction is preferably from 0.3 .mu.m to 0.4 .mu.m.
An electrode is required to derive the photoexcited current to the exterior from the semiconductor photodiode. This electrode is commonly formed by application (deposition, printing, etc.) of an electrode material to a predetermined region of the semiconductor substrate and by subsequent heat treatment. In this case, inversion and alloying (including a silicide reaction) tend to occur in the semiconductor surface layer beneath the electrode. In order that the electrode showing satisfactory electrical properties can be formed on the semiconductor surface layer, the semiconductor surface layer has preferably a low sheet resistance and/or a sufficient thickness. Since the portion of the p-n junction beneath the light receiving surface is preferably as shallow as possible as described above, the general profile of the p-n junction would preferably be such that it has a locally changing depth. The region where the electrode is to be formed on the semiconductor substrate surface will be referred to hereinafter as an electrode forming part.
Unexamined Japanese Patent (Laid-Open) Publication No. 59-79580 proposes a method of manufacturing a photodiode comprising the steps of vapor-phase diffusing an impurity into a semiconductor substrate, patterning an oxide film formed subsequently and including the impurity to leave the oxide film only on an electrode forming part, and, then subjecting it to heat treatment, thereby driving in the impurity. The impurity diffuses more from the portion having the remaining oxide film, thereby forming a p-n junction whose depth changes locally and which is deeper at the electrode forming part than at the light receiving part.
According to the proposed method, two steps of impurity doping are required, and a patterning step between these two steps is also required.
Therefore, a method of manufacturing a high-efficiency photodiode by simplified steps is demanded.